thunderstorms and lightning - university of...

Thunderstorms and Lightning

• Thunderstorm development and Charge separation

• Lightning • Transient Luminous Events (TLEs such as

Sprites and Jets)• But first: How Atmospheric Electricity gave

saved the Union

Franklin chimes

grounded

To lightning rod

ringer

Benjamin Franklin

Ben Franklin

• 5 letters to London Royal Society in 1750• Sentry Box experiment conducted May 10,

1752• Reported 3 days later at Royal Society:

On 10 May 1752, as a thunderstorm passed over the village of Marly-la-Ville, a retired French dragoon, acting on instructions from naturalist Thomas-François Dalibard, drew sparks from a tall iron rod that had been carefully insulated from ground (see figure 1). The sparks showed that thunderclouds are electrified and that lightning is an electrical discharge. (reference: Krider,2006)

Lightning Rods• First protection system• In the 1753 issue of Poor Richard's Almanac, Franklin published a method for

protecting houses from lightning damage: • It has pleased God in his Goodness to Mankind, at length to discover to them

the Means of securing their Habitations and other Buildings from Mischief by Thunder and Lightning. The Method is this: Provide a small Iron Rod (it may be made of the Rod-iron used by the Nailers) but of such a Length, that one End being three or four Feet in the moist Ground, the other may be six or eight Feet above the highest Part of the Building. To the upper End of the Rod fasten about a Foot of Brass Wire, the Size of a common Knitting-needle, sharpened to a fine Point;

• In the next 10-15 years, Europe and America sprouted lightning rodseverywhere: Franklin became a great hero. He was awarded honorary degrees from awarded honorary doctor's degrees by St. Andrews (1759) and Oxford (1762) , giving him access to the courts of Europe.

• You could say that Atmospheric Electricity Experiments saved the United States!

Rain Shaft

anvil

Updraftregion

Mature Thunderstorm

Florida Thunderstorms

• Go to FloridaThunderstorm.mov

How do thunderstorms charge up?

• Charge is separated when two kinds of ice have a collision

• kinds of ice: snowflakes and ice crystals, soft ice, hail stones, frozen rain (sleet), ...

• Each has different surface electrical properties

ice crystal

-

+hail

updraft

Intro to TLEs

• TLE: Transient Luminous Events• See ISS video:

http://www.esa.int/esaCP/SEM4HV4Y3EE_index_1.html

• Sprites, Jets, Elves, Halos … • Current Experiments: RHESSI,

Sprite scenario

Winckler, et al

stenbaek-nielsen, GRL, 2000

Hardman, et al, GRL, 2000

1. The cloud charges up before the lightning discharge inducing a negative shielding layer

2. The positive CG removes positive charge but the negative shielding layer remains over a much longer time scale

The Quasi-Electrostatic Field Model

3. The negative shielding layer remains after the discharge causing polarization in the atmosphere and a quasi-static E-field. This can me likened to a giant parallel plate capacitor as shown above. This strong E-field causes electrical breakdown producing sprites.

The Quasi-Electrostatic Field

Cummer and Stanley, GRL, 99

Problems with the models, as shown by Cummer & Stanley:

Cummer

quote

The data (previous slide) show that vertical charge moment changes of 800 – 1100 CKm in 2 – 4 ms are associated with sustained emissions at altitudes as low as 50 km. This is a factor of 2 to 10 times smaller than the vertical lightning charge moment changes required to produce runaway and conventional breakdown at 50 km, respectively.

These observations are consistent with the generation of significant mesospheric electric fields by horizontal currents in some sprites, but we cannot rule out other processes as the explanation for the observations(Cummer et al, 1999)